Transcontinental Triangulation

The Transcontinental Triangulation was authorized by Congress in 1871 to measure a coast-to-coast arc along the 39th parallel. The work took nearly three decades, and resulted in the first accurate measure of the continent's width, and one of the largest arcs ever measured. Among the many results of the triangulation was a new calculation of the figure of the earth and the selection of the Clarke 1866 spheriod as the best fit to the continental United States.

The arc also provided the foundation for a national datum, and anchored the other great triangulation projects to the nation's heartland. Prior to the transcontintal triangulation, regional networks were based on local datums. When tied to the transcontinental arc, the local networks became a national network upon which a datum could be based. The station Meade's Ranch was first surveyed in the transcontinental triangulation and ultimately became the fixed point for the NAD27 datum adjustment.

The triangulation consisted of 10 base line surveys and 11 separate triangulation series connected to the baselines. In the East, portions of the earlier Coast and Geodetic Surveys from the 1840's were incorporated into the network. In Illinois, the Olney base line from the U.S. Lake Survey was incorporated. The majority of the network was surveyed specifically for the transcontinental arc, and CGS officers were responsible for the annual survey parties that slowly made their way across the nation. The last horizontal angle measurements were completed in 1896 but the work of computing the adjustments to the network took another 4 years, and the final report was published in 1900. In all, 55 officers of the CGS participated in the project.

The project also included hypsometry work to acquire elevation data, which was needed for reducing the observed angles to sea level and computing the shape of the geoid. In addition, astronomic observations provided latitude and azimuth data. Telegraphic longitude observations were obtained at 39 of the stations, and along with the latitude and azimuth observations, were used to compute the deflection of the vertical.

The accuracy of the work was such that the total computed length of the 39th parallel arc over 4,224 kilometers was believed to be accurate to within 26 meters. The earliest base line, the Kent Island base line of 1844, had been measured to within +/- 7 cm over 8.8 km. The last base line measurement, the Versailles base line of 1897, was an order of magnitude better, with an accuracy of +/- 8 mm over 7.6 km.

The surveyors sometimes endured severe hardships to accomplish their mission. In the mountains of the west the stations were often located on the highest peaks and the teams endured high winds, extreme cold, and dangerous lightning. It was necessary to transport delicate equipment over roadless territory, and then build trails up the sides of the mountains before the equipment could be mounted. Often in that rugged country, only one or two stations could be occupied in a season. The total number of stations included in the network was 350.

Most of the stations that were occupied during the triangulation are still to be found in the NGS database. Details of the base lines and series are found below.